The invention relates to the field of transistors, and in particular to a thin film transistor (TFT) having high-K gate dielectrics.
Thin film transistor (TFT) technology on plastic substrates offers many potential advantages not only with respect to displays but also to wide area digital logic circuitry, electronic-paper and books, wearable electronics, sensors, photodetectors, actuators. Room temperature fabricated Organic-TFTs (OTFTs) and ZnO-based transparent-TFTs can contribute to reduced cost by substituting inexpensive plastic substrates for relatively high cost Si wafers or flat glass substrates utilized today.
TFT-type sensors are undoubtedly of great importance for the development of microelectronics-based smart sensing systems. Several parameters of hybrid OTFT (organic thin film transistors)/Metal-Oxide TFT (semiconducting metal-oxide thin film transistors) such as saturation current, on/off current, threshold voltage, and sub-threshold voltage slope can vary in response to various gases and volatile organic compounds. Advantages of TFT-based sensors include high sensitivity and selectivity, resulting from special features of organics e.g. their specialized bonding sites, coupled with the ability to use flexible substrates and low temperature processing leading to low manufacturing costs. In order to form electronic noses (e-noses), which are designed to detect and distinguish between many complex odors, large arrays of sensors are required. While this can be done in principle with other types of sensors such as SAW (surface acoustic wave) devices, quartz crystal microbalances (QCM), organic/inorganic resistors, their size and complexity often make this impractical. The ability to assemble gas sensitive TFTs into large arrays is therefore an important feature.
Such TFT-type sensors benefit by ease of integration with associated transducer or signal processing circuitry, as well as, into fabrics. Also, the development of ultra low-power TFT compatible gas sensor arrays can be a key component for further technologies. To date, the operating voltages, between 10 and 100 V, of TFTs with low-K gate dielectrics on plastic substrate are commonly too high, especially for portable, battery-powered sensing device applications, e.g. medical monitoring and/or toxic chemical sensors integrated within military uniforms.
ZnO is also known to have the capacity to detect ultraviolet (UV) photons. ZnO-based TFTs can be used for photodetecting elements. Photodetecting devices with low cost and low voltage operation (<5V) will have benefits. High-K gate oxide growth by a near room temperature process on glass or plastic substrates is essential.
Another interesting application area utilizes CMOS technology based on a combination of OTFT (organic thin film transistors)/Metal-Oxide TFT (semiconducting metal-oxide thin film transistors) with p-channel and n-channel as active channels, respectively. Pentacene is a well studied p-type semiconductor material. Undoped zinc oxide (ZnO) has an electron channel mobility which can support high drive currents and fast operating speeds. Alternatively, InGaO3, with field effect mobility of 80 cm2/Vs, could be used instead of ZnO.